In the cryogenic rectification of feed air to produce one or more products such as oxygen, often it is desired that product be recovered as elevated pressure gas. One way of achieving this is to operate the column or columns of the cryogenic air separation plant at elevated pressure and recover elevated pressure gaseous product directly from the distillation column. However, such a system is generally disadvantageous because the elevated pressure within the column burdens the separations. Preferably the final separation within a column is carried out at a relatively low pressure and, if elevated pressure gaseous product is desired, the product is withdrawn from the column and its pressure increased prior to recovery.
For the recovery of elevated pressure gaseous product, the product may be withdrawn from the column as gas and then compressed to the desired pressure. However, it is generally more preferable that the product be withdrawn from the column as liquid, pumped to the desired pressure, and then vaporized in a product boiler to produce the desired elevated pressure gas.
Typically the product boiler is a pool boiler heat exchanger which is separate from other heat exchangers of the system. This arrangement is very effective but is costly. It is desirable that the product boiler be integrated with the primary heat exchanger of the system and such arrangements are known. However, in some situations the integration of the product boiler with the primary heat exchanger may lead to a boiling to dryness problem wherein residual hydrocarbons may concentrate in oxygen creating a flammability issue and potential danger.
Accordingly, it is an object of this invention to provide a cryogenic rectification system for producing elevated pressure gaseous product employing a product boiler integrated with the primary heat exchanger which enables avoidance of any hazard due to boiling to dryness.